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Trees in Australia's tropical rainforests have become the first worldwide by shifting from acting as a carbon sink to becoming a source of emissions, due to rising heat extremes and arid environments.

The Tipping Point Discovered

This significant change, which affects the stems and limbs of the trees but does not include the root systems, started around 25 years ago, as per recent research.

Trees naturally store carbon as they develop and release it when they decompose. Generally, tropical forests are regarded as carbon sinks – absorbing more CO2 than they emit – and this uptake is expected to grow with rising atmospheric concentrations.

However, close to five decades of data gathered from tropical forests across Queensland has revealed that this essential carbon sink could be under threat.

Study Insights

Approximately 25 years ago, tree trunks and branches in these forests became a net emitter, with more trees dying and insufficient new growth, as the study indicates.

“This marks the initial rainforest of its kind to display this sign of transformation,” stated the lead author.

“It is understood that the humid tropical regions in Australia exist in a slightly warmer, drier climate than tropical forests on different landmasses, and therefore it could act as a future analog for what tropical forests will experience in global regions.”

Worldwide Consequences

A study contributor mentioned that it remains to be seen whether Australia’s tropical forests are a precursor for other tropical forests globally, and additional studies are required.

But if so, the findings could have major consequences for international climate projections, carbon budgets, and environmental regulations.

“This research is the initial instance that this tipping point of a switch from a carbon sink to a carbon source in tropical rainforests has been identified clearly – not just for one year, but for 20 years,” remarked an authority on climate science.

Worldwide, the portion of carbon dioxide absorbed by forests, trees, and plants has been relatively constant over the past few decades, which was assumed to continue under many climate models and strategies.

But should comparable changes – from sink to source – were observed in other rainforests, climate forecasts may understate heating trends in the future. “This is concerning,” he added.

Ongoing Role

Even though the equilibrium between growth and decline had changed, these forests were still playing an important role in soaking up CO2. But their reduced capacity to take in additional CO2 would make emissions cuts “a lot harder”, and necessitate an accelerated transition away from fossil fuels.

Research Approach

This study drew on a distinct collection of forest data starting from 1971, including records monitoring approximately 11,000 trees across numerous woodland areas. It focused on the carbon stored in trunks and branches, but not the gains and losses in soil and roots.

An additional expert emphasized the value of collecting and maintaining long term data.

“We thought the forest would be able to store more carbon because [CO2] is increasing. But looking at these decades of recorded information, we find that is incorrect – it allows us to compare models with actual data and improve comprehension of how these systems work.”